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Defluoridation Technology

Most of the defluoridation methods are based on the principle of adsorption or ion exchange, such as the use of lime softening, alum, activated alumina, bone char, synthetic hydroxyapatite, ion exchange resins, reverse osmosis etc. The shortcomings of most of these methods are: high initial cost of the defluoridation plant, high operation and maintenance cost, low fluoride removal capacity, lack of selectivity for fluoride, undesirable effects on water quality during defluoridation and short life span of the filter. Since 1998, Catholic Diocese of Nakuru Water Quality (CDNWQ) has been using the bone char for defluoridation. In this technology fluoride is removed from the water by passing it through the bone char filter material where fluoride is adsorbed in the gravels, letting the fluoride free water from the water. The bone char method was selected as the most appropriate for the following reasons:  

•   The method is safe  

•   All the fluoride can be removed  

•   The filters are easy to use  

•   No chemicals should be added  

•   All materials used can be replaced locally

 •   The filters are produced locally

This method is considered to be an adsorption or ion exchange process. Though bone char method is simple and inexpensive, there have been continuous improvements since regeneration of bone char was not easy and the lifespan of the filter was relatively short. To overcome these drawbacks, CDNWQ adopted a contact precipitation method of fluoride removal which is more effective starting with first field testing in 2006.

How contact precipitation works? In this method ion exchange and surface adsorption on bone char are regarded as supplementary reactions rather than principal mechanism. The chemical processes involved are assumed to be fluoride adsorption on bone char surface, dissolution of calcium and phosphate and further precipitation of a combination of Calcium Fluoride and Fluorapatite on bone char surface.

The contact precipitation method has recently been adopted as a method of defluoridation by CDNWQ because of the following addition benefits:  

•   Life span of is prolonged up to 6 times  

•   Operation and maintenance of the filters is easy

Comparisons of bone char only and contact precipitation technology

A comparison of the two technologies (bone char (BC) and contact precipitation (CP) technology) was carried out on laboratory scale under the same conditions i.e. same inlet fluoride concentration, same flow rate. For contact precipitation, a ratio of 3:1 for Ca-PO4 pellets: bone char was used and for bone char technology only bone char was used. In the experiments, PVC columns with 260 ml filter material, Pellet size:2-6.3mm; Bone Char(BC) size:1-4mm at a flow rate 10Bv/day with raw water concentration 6mgF/l were utilized .

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